<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<style>
    body {
        padding: 50px;
    }
    
    .mydiv {
        height: 200px;
        width: 200px;
        border: 1px solid #ccc;
        margin-bottom: 20px;
        display: inline-block;
    }
    
    .case1 {
        background: #58a;
        background: linear-gradient(-45deg, transparent 15px, #58a 0);
    }
    
    .case2 {
        background: #58a;
        background: linear-gradient(-45deg, transparent 15px, #58a 0) right, linear-gradient(45deg, transparent 15px, #655 0) left;
        background-size: 50% 100%;
        background-repeat: no-repeat;
    }
    
    .case3 {
        background: #58a;
        background: linear-gradient(135deg, transparent 15px, #58a 0) top left, linear-gradient(-135deg, transparent 15px, #58a 0) top right, linear-gradient(-45deg, transparent 15px, #58a 0) bottom right, linear-gradient(45deg, transparent 15px, #58a 0) bottom left;
        background-size: 50% 50%;
        background-repeat: no-repeat;
    }
    /* 弧形切角 */
    
    .case4 {
        background: #58a;
        background: radial-gradient(circle at top left, transparent 15px, #58a 0) top left, radial-gradient(circle at top right, transparent 15px, #58a 0) top right, radial-gradient(circle at bottom right, transparent 15px, #58a 0) bottom right, radial-gradient(circle at bottom left, transparent 15px, #58a 0) bottom left;
        background-size: 50% 50%;
        background-repeat: no-repeat;
    }
</style>

<body>
    <div class="mydiv case1"></div>
    <div class="mydiv case2"></div>
    <div class="mydiv case3"></div>
    <div class="mydiv case4"></div>
</body>

</html>